Gasoline typically includes a mixture of hydrocarbons ranging from higher volatility butanes (C4) to lower volatility C8 to C10 hydrocarbons. When vapor pressure increases in the fuel tank due to conditions such as higher ambient temperature or displacement of vapor during filling of the tank, fuel vapor flows through openings in the fuel tank. To prevent fuel vapor loss into the atmosphere, the fuel tank is vented into a canister that contains an adsorbent material such as activated carbon granules (“evap” canister).
The fuel vapor is a mixture of the gasoline vapor (referred to in this description also by its main component, hydrocarbon vapor) and air. As the fuel vapor enters an inlet of the canister, the hydrocarbon vapor is adsorbed onto activated carbon granules and the air escapes into the atmosphere. The size of the canister and the volume of the adsorbent activated carbon are selected to accommodate the expected gasoline vapor generation. After the engine is started, the control system uses engine intake vacuum to draw air through the adsorbent to desorb the fuel. The desorbed fuel vapor is directed into an air induction system of the engine as a secondary air/fuel mixture. One exemplary evaporative control system is described in U.S. Pat. No. 6,279,548 to Reddy, which is hereby incorporated by reference in its entirety.
When the gasoline tank is filled, fuel vapor accumulates in the canister. The initial loading may be at the inlet end of the canister, but over time the fuel vapor is gradually distributed along the entire bed of the adsorbent material. After the engine is started, a purge valve is opened and air is drawn through the canister. The air removes fuel vapor that is stored in the adsorbent material.
One problem encountered by such a system has been vapor breakthrough, or hydrocarbon emissions from the vented vapor adsorption canister, which is often referred to as canister bleed emissions. Such emissions may be, for example, about 20 mg hydrocarbons per day.
The problem of bleed emissions is particularly acute in hybrid vehicles. Hybrid vehicles combine a gasoline fueled internal combustion (IC) engine and an electric motor for better fuel economy. As will be appreciated, in a hybrid vehicle, the internal combustion engine is turned off nearly half of the time during vehicle operation. Because the purging of an evap canister takes place only during operation of the internal combustion engine when the desorbed vapor can be consumed in engine combustion, the evap canister purging with fresh air occurs less than half of the time the hybrid vehicle is running. Thus, although a hybrid vehicle generates nearly the same amount of evaporative fuel vapor as does a conventional vehicle, its lower purge rate may be insufficient to clean the adsorbed fuel out of the evap canister, thereby resulting in higher evaporative bleed or breakthrough emissions.
Accordingly, meeting the zero evap standard for hybrid vehicles is turning out to be particularly difficult. Some prior artisans have attempted to reduce breakthrough emissions by reducing the amount of vapor that is generated in the tank or that escapes the tank. For example, some hybrid systems use an expensive and complex semi-bladder tank for reducing tank vapor generation. The reduced tank vapor generation results in the need for a smaller evap canister, which in turn, can hopefully be sufficiently purged with the hybrid's lower purge air volume.
Regardless of its size, evaporative fuel stored in the evap canister needs to be purged and consumed in engine combustion. If the canister in not purged with a sufficient volume of purge air, as is problematic with hybrid vehicles, the canister bleed or breakthrough emissions will increase significantly once the canister is saturated.
Co-pending U.S. patent application Ser. No. 10/303,556, filed Nov. 25, 2002, which is hereby incorporated by reference in its entirety, describes a method and system for evaporative emission control in which bleed emissions from the evap canister are adsorbed by activated carbon fibers in a secondary canister (or further chamber of the evap canister) referred to generically as a “scrubber.” The system may be used in a conventional automotive vehicle having only an internal combustion engine or in a hybrid vehicle that includes both an internal combustion engine and an electric motor. In an embodiment of the zero evap system, the system comprises a three-chamber canister and a small auxiliary hydrocarbon scrubber. The scrubber preferably comprises either a carbon monolith or an activated carbon fiber felt. A presently available scrubber for use with the present invention is a MeasWestvaco of the type described in U.S. Pat. No. 6,540,815. As will be appreciated, the composition of the adsorbents in the canister and scrubber are based on the composition of hydrocarbons being absorbed. The choice of materials for heated purge as described herein is within the ability of one of ordinary skill in the art armed with the present specification.
Typically, evaporative fuel vapor consists of about 50% hydrocarbons in the C4 to C10 range and the balance of the mixture being air. The main canister's activated carbon traps almost all of the hydrocarbons with the exception of small amounts of the more volatile components (C4 and C5) which escape with the air. The hydrocarbon scrubber is provided to trap these C4 and C5 components that escape from the main canister. This system may be effective in reducing breakthrough emissions to almost zero. However, both the scrubber and primary canister need to be purged with a sufficient amount of air to hinder bleed emissions. Accordingly, there remains a need in the art for effective purging of evap systems, and in particular, evap systems on hybrid vehicles.